fix(commit): keep conventional prefixes lowercase

[noyobo]

What & why

Typing a conventional prefix such as fix: in the commit summary was rewritten to Fix: by the field's sentence capitalization. That breaks the commit-message convention, so the summary and description now set autoCapitalize="off". Spell check stays on.

This covers the working-copy commit box and the inspector message editor.

Screenshots

No layout or color change. The check is that fix: stays lowercase after leaving the field, and a misspelled word is still marked while the field is focused.

Checklist

• pnpm typecheck && pnpm test pass
• cargo fmt --check && cargo clippy --all-targets -- -D warnings && cargo test pass (if Rust changed)
• New engine functions have integration tests in tests/git_engine.rs
• Mutating operations go through the undo tracked() wrapper where undoable
• Colors use design tokens only (no hex in components)
• CLAUDE.md updated if conventions/architecture changed
• Commits carry no trailers from coding tools (Co-authored-by: <tool>, Generated with …)